The present invention relates to a strain gauge for the pressure sensor and method for manufacturing the same, and especially relates to an improved strain gauge and method for manufacturing which obtains high stabilization.
Japanese laid-open patent publication No. 174844/1985 published on Sept. 9, 1985 discloses a conventional strain gauge prepared on a diaphragm. Referring now to FIG. 3, the conventional strain gauge is explained. As shown in FIG. 3, two pair of the strain gauges A and B are provided on a central part and circumference part of a diaphragm 2. In order to convert the applied pressure to the electric signal linearly, the respective pair of strain gauges A and B are connected together as a bridge circuit.
The strain gauges A and B are made of an amorphous alloy film of amorphous Ni.sub.a Si.sub.b B.sub.c, where symbols a, b and c have ranges as 60.ltoreq.a (atomic%).ltoreq.74, 16.5.ltoreq.b (atomic%), 26.ltoreq.b+c (atomic%).ltoreq.40. The strain gauges A and B are provided onto the diaphragm directly by using sputtering method or vacuum deposition method.
A conventional alloy of amorphous Ni.sub.a Si.sub.b B.sub.c has a very small absolute value of temperature co-efficient of resistance smaller than 50 ppm/.degree.C. Therefore, the conventional strain gauge using the conventional alloy is stabilised against variation in temperature. Also, the conventional strain gauge is placed onto the diaphragm directly. Therefore, an adhesive need not to be used in order to attach the strain gauge on the diaphragm so that the mechanical reliability of the strain gauge is excellent. Further, the conventional alloy is a non-magnetic substance. Therefore, the strain gauge is hardly affected by the external magnetic field. Furthermore, the conventional alloy has a high resistivity and has a small cross-sectional area. Therefore, resistance of the gauge to be established is high.
However, in the conventional alloy of amorphous NiSiB, the usuable range as the strain gauge is narrow, and more particularly, endurance at higher than 80.degree. C. is not obtained.
Referring now to FIGS. 4 and 5, wherein problems of the conventional strain gauge are explained. From now on, a transit ratio is defined as the ratio between one output voltage of the bridge circuit under applying a maximum usable pressure and the other output voltage of the bridge circuit under applying no pressure to the diaphragm. The relationship between a transit ratio on a vertical axis and elapsed time on a horizontal axis is shown in FIGS. 4 and 5. FIG. 4 shows the relationship detecting under 60.degree. C., and FIG. 5 shows the relationship detecting under 120.degree. C.
As shown in FIG. 4, the transit ratio shows a transition within about 0.2% after 15 days under 60.degree. C. Contrary, as shown in FIG. 5, the transit ratio shows a transition about 7% after 15 days under 120.degree. C. Further, under 120.degree. C., the strain gauges are separated from the diaphragm of almost all samples after 14 days, and then the transit ratio changes greatly. This phenomena shows a of the heat-resistance of the conventional strain gauge using the alloy of amorphous NiSiB.